1. Field of the Invention
The present invention relates to color image forming apparatuses, and more particularly to a color image forming apparatus suitable for an image forming by electrophotography.
2. Background of the Invention
In recent years, in image formation by electrophotography, an image forming apparatus has appeared where an original image (document) of full color is used and a copy of full color is obtained. A multicolor image readily not only enables one to reproduce a person, a still life, a landscape or the like but it also allows for the incorporation of a great amount of additional information in one recording image and makes the process significantly favorable for a drawing, a table or the like.
Under such circumstances, various methods and apparatuses for multicolor image forming have been proposed.
For example, a method has been disclosed where a plurality of latent image forming means and a plurality of developing means are arranged around a rotating drum-shaped photosensitive member, and the latent image forming and the developing are repeated, whereby visible images that are different from each other in color are formed in a superposed manner on the drum-shaped photosensitive member and the superposed images are transferred at one time onto a recording paper (Japanese Pat. Laid Open Nos. 52-106743, 56-144452, 58-79261, 61-170754).
Another method has been disclosed where one latent image forming means and a plurality of developing means are arranged around a rotating drum-shaped photosensitive member, and the latent image forming and the developing regarding one color are performed per each rotation of the photosensitive member, thereby multicolor visible images are formed by a plurality of rotations of the photosensitive member and the multicolor images are transferred at one time onto a recording paper (Japanese Pat. Laid Open Nos. 60-76766, 60-95456, 61-170754).
In the method of the former, when color to be reproduced is a full color composed of yellow, magenta, cyan and black if necessary, the latent image forming means and the developing means respectively are the same in number as the colors used and must be arranged around the photosensitive member causing the diameter of the photosensitive member to become large. Furthermore, during the latent image forming, in order to assure registration of each color separation latent image, a control device to improve the write accuracy of the latent image forming means, such as a laser, LED, LCS or the like, are necessary. On the other hand, since the read scan of one time is sufficient, registration of read is good but an image memory of large capacity is necessary.
In the method of the latter, since only one latent image forming means is used, the apparatus becomes compact in comparison to the method of the former. Particularly since the latent image forming means can be used commonly, this method is advantageous in the registration of a latent image. However, the photosensitive member must be rotated the same number of times as the colors and therefore the speed of multicolor image forming becomes slow. On the other hand, when the image memory is not provided, the read scan is performed three or four times, thereby the registration of the read means becomes a problem in contrast to the method of the former.
In a conventional image forming apparatus where a plurality of laser raysare individually scanned and a composite image is formed on a photosensitive member, rotary polygon mirrors of the scanning means and f-.theta. lenses for correcting the spot incident distribution of laser rays and the focus variation are necessitated by the number of the laser rays. Moreover, the spot incident positions of the plurality of laser rays must be suitably registered. The exact and stable registration is very difficult because the rotary polygon mirrors include variations in the rotational speed and the surface precision.
In order to eliminate the above-mentioned problems, an image recording apparatus is proposed in Japanese Pat. Laid Open No. 56-161566 where when two laser beams are incident at different angles to the same surface of one rotary polygon mirror and the scanned laser rays are incident to a photosensitive member through a common f-.theta. lens, a laser ray being incident slantwise to the rotary axis of the rotary polygon mirror is made incident through a cylindrical lens correcting distortion of the scanning line. Also an image recording apparatus is proposed in Japanese Pat. Laid Open No. 58-95361 where two laser beams are incident respectively to different surfaces of one rotary polygon mirror, and the respectively scanned laser rays are incident through a f-.theta. lens to separate electrophotographic sensitive members, and further toner images formed on the electrophotographic sensitive members are transferred onto a transfer paper and composed. Apart from the point that laser rays in the latter apparatus are incident to separate electrophotographic sensitive members and toner images are formed and therefore the registration is difficult, optical systems of both recording apparatuses will be compared. In the former, since two laser rays are incident to the same surface of the rotary polygon mirror, although a problem of difficulty of the registration due to variation of the surface precision is small in comparison to the latter where laser rays are incident on different surfaces, the problem cannot be entirely eliminated because of the difference in incident angles and there is another problem in that the cylindrical lens for correction is necessary. In the latter, since two laser rays can be incident respectively orthogonal to the rotary axis of the rotary polygon mirror, a cylindrical lens is not required. However, since two laser rays are incident to different surfaces as above described, difficulty in registration due to variation in the surface precision becomes large in comparison to the former. Also a problem exists in that a f-.theta. lens is required for each laser ray, and further an essential problem exists in that the scanning directions of two laser rays are reverse to each other.
Also in a conventional color image forming apparatus as above described, developing devices can be installed and detached individually, and when all developing devices are installed or detached for inspection or maintenance, a problem exists in that much labor is required. As a color image forming apparatus to eliminate the problem, the present inventors previously invented an apparatus having one electrostatic image forming means where all of plural developing devices are enclosed in one case and therefore can be installed and detached together with the case. In this constitution, however, when the developing devices become 3-4 in number, the case becomes large and heavy and therefore a problem exists in that the case is inconvenient for installing or detaching and also the setting accuracy of the developing devices is liable to be deteriorated. On the other hand, in an apparatus having a plurality of electrostatic image forming means, since the distance between the developing devices becomes longer, if all developing devices are enclosed in one case and then installed or detached together with the case, the above-mentioned problem will become even more serious.